1. Field of the Invention
The present invention relates to a serial data receiving apparatus which is appropriate to receiving serial data having an inferior SN ratio as in a radio data communication through weak radio waves.
2. Description of the Prior Art
As a conventional example of such a serial data receiving apparatus, an asynchronous system receiver has been known. The asynchronous system is arranged so that as shown in FIG. 9, a trailing edge of a signal (point P) is assumed as a signal start point, and an sampling signal is input at the centers (C1, C2, C3, . . . ) of the bits where predetermined lag times (t1, t2, t3, . . . ) respectively have passed from the signal start point. The P point means a point where the bit synchronization is established.
In the conventional arrangement of the serial data receiving apparatus, however, it is impractical to establish the bit synchronization in the condition that the SN ratio of the data is so inferior as often bringing about noise on the data, because it is impossible to determine the actual trailing edge of the signal from the trailing portion resulting from the noise. Furthermore, though the use of a kind of means may establish the bit synchronization, the erroneous data inputting cannot be prevented if noise is located at the input point of the sampling signal.
For the conventional means for overcoming such a shortcoming, a noise filter has been often used. The filter serves to separate the noise from the signal in light of the frequency. It means that the ideal noise filter is arranged to enter the frequency components of the data itself and block the other frequency components. However, even such an ideal noise filter is incapable of preventing the noise having the same frequency components as the data. Hence, the use of the noise filter is not so effective in overcoming the shortcoming resulting from the noise.
As another method, there has been proposed a method for separating the signal from the noise by overlapping signals on time. In the field of satellite communication, there has been practically employed the method where a satellite transmits a signal again and again and a receiving apparatus on the earth adds the same signals transmitted from the satellite again and again and computes an average value of the added signals. The use of this method makes it possible to practically eliminate the adverse effect of the random noise components and restore a vivid signal.
To realize this method, a large-volume memory is required for temporarily storing the data transmitted from the satellite. Further, a high-speed processor is required for adding and averaging these data signals. In case that the budget disallows the use of an expensive high-speed processor, it takes a long time to do the computation. Hence, it is impractical to apply this technique to domestic equipment in light of the cost, and this technique cannot realize a real-time signal-transmitting facility like a remote control unit provided in the domestic equipment. The additional disadvantage of this technique is that since the same data signal is fired to the same space again and again for transmitting one piece of data, the transmitted data occupies the space during the signal-transmitting time and no other data is allowed to be transmitted. FIG. 10 is a sketch illustrating a remote control system of a spontaneous gas water heater to which the present invention applies. A gas water heater 101 is normally installed outside of wall or in a machine chamber. In the gas water heater 101, the gas supplied from the gas supply pipe 102 is combusted for heating the water supplied from a water pipe 103. The hot water is supplied through a hot water pipe 104 to a shower 107 located in a bathroom 105 or a faucet 108 provided in a kitchen 106. The bathroom 105 or the kitchen 106 provides remote control units 109 and 110 by which the hot water temperature is adjusted. The operation signals of the remote control units 109 and 110 are converted into radio signals 111, 112 and then are transmitted to the gas water heater 101 located behind walls 113 and 114. Conversely, the signal indicating a driving state signal of the gas water heater 101 is transmitted as a radio signal (not shown) to the remote control units 109 and 110. The driving state is indicated on an indicator. In case that the walls 113 and 114 contain a certain kind of metal material or the gas water heater 101 is located far apart from the bathroom 105 and the kitchen 106, the radio signals 111 and 112 become attenuated until they reach the bathroom 105 or the kitchen 106. This results in making the SN ratio of the signal inferior when the receiver (not shown) of the gas water heater 101 receives the signal.
As set forth above, the present invention is designed in the consideration that the receiver for receiving a serial data signal containing noise components on real time is applied to a relatively inexpensive commodity. For example, the present invention may apply to a home automation system including a security facility or a portable telephone in addition to the aforementioned remote control system of the domestic equipment or the home.